(a) Field of the Invention
The present invention relates to a power saving mode control method and device in a wireless portable Internet system. More specifically, the present invention relates to a power saving mode control method and device for using a power saving mode and a sleep mode for reducing power consumption of mobile terminals in a mobile communication system and a wireless Internet system.
(b) Description of the Related Art
A wireless portable Internet is a next generation communication scheme for further supporting mobility for short range data communication schemes which use fixed access points, such as the conventional wireless LAN. Various standards for the wireless portable Internet have been proposed, and the international standard of the portable Internet has progressed through the IEEE 802.16e.
FIG. 1 shows a diagram of the wireless portable Internet system.
As shown, the wireless portable Internet system includes a subscriber station (SS) 111 in cells 110 and 120, base stations 112 and 121 for performing radio communication with the SS 111 in the cells 110 and 120, routers 131 and 132 connected to the base stations 112 and 121 through a gateway, and the Internet 140. A personal computer is also provided as a terminal node 150.
The wireless LAN such as the conventional IEEE 802.11 provides a data communication scheme for allowing short-range wireless communication with reference to a stationary access point, and it does not provide mobility of the SS but rather it supports the short-range data communication in a wireless manner instead of on the cable basis.
The wireless portable Internet system driven by the IEEE 802.16 group guarantees mobility and provides a seamless data communication service when the SS 111 shown in FIG. 1 moves to another cell from a cell.
The IEEE 802.16 basically supports the metropolitan area network (MAN), and represents an information communication network covering an intermediate area between the LAN and the WAN.
Therefore, the wireless portable Internet system supports a handover of the SS 111 in a like manner of the mobile communication service, and assigns dynamic Internet protocol (IP) addresses according to movement of the SS 111.
In this instance, the SS 111 communicates with the base stations 112 and 121 through the orthogonal frequency division multiple access (OFDMA) system, which is a modulation and multiple access scheme having combined the orthogonal frequency division multiplexing (OFDM) scheme which uses the frequency division multiple access (FDMA) scheme for using a plurality of subcarriers of orthogonal frequencies as a plurality of subchannels, and the time division multiple access (TDMA). The OFDMA system is essentially resistant to the fading phenomenon generated on the multi-paths, and has high data rates.
Also, the IEEE 802.16 has adopted the AMC (adaptive modulation and coding) scheme for adaptively selecting a modulation and coding scheme according to a request and an acceptance between the SS 111 and the base stations 112 and 121.
FIG. 2 shows a hierarchical structure of the wireless portable Internet system.
As shown, the hierarchical structure of the wireless portable Internet system of the IEEE 802.16e includes a physical layer 210 and a media access control (MAC) layer 220, and the physical layer 210 is connected to the MAC layer 220 through a service access point (SAP).
The physical layer 210 performs radio communication functions executable on the general physical layer, such as modulation/demodulation, and coding.
The wireless portable Internet system does not have layers classified according to their functions, but allows a single MAC layer to perform various functions, differing from the wired Internet system.
Regarding sublayers according to functions, the MAC layer includes a privacy sublayer 221, an MAC common part sublayer 222, and a service specific convergence sublayer 223.
The service specific convergence sublayer 223 performs a payload header suppression function and a quality of service (QoS) mapping function in the case of seamless data communication.
The MAC common part sublayer 222, which is the core part of the MAC layer, performs a system access function, a bandwidth allocation function, a connection establishment and maintenance function, and a QoS management function.
The privacy sublayer 221 performs a device authentication function, a security key exchange function, and a data encryption function. The device is authenticated by the privacy sublayer L21, and the user is authenticated by an upper layer (not illustrated) of the MAC.
FIG. 3 shows a diagram of a connection configuration between a base station (BS) and an SS in the wireless portable Internet system.
The MAC layer 220a of the SS and the MAC layer 220b of the BS have a connection therebetween.
The phrase connection represents not a physically connected relation but rather a logically connected relation, and it is defined to be a mapping relation between MAC peers of the SS and the BS in order to transmit traffic of a single service flow.
Therefore, parameters or messages defined with respect to the connection represent the functions between the MAC peers, and in reality, the parameters or the messages are processed, are converted into frames, and are transmitted through the physical layers, and the frames are parsed and the functions which correspond to the parameters or the messages are executed on the MAC layer.
The MAC messages transmitted through the connection include: a connection identifier (CID) which is an MAC layer address for identifying connections; an MAP for defining a symbol offset, a subchannel offset of bursts time-divided by the SS in a downlink/uplink, a number of symbols, and a number of subchannels of allocated resources; and a channel descriptor for describing characteristics of the physical layer according to the characteristics of the downlink/uplink (a downlink channel descriptor will be referred to as a DCD and an uplink channel descriptor will be referred to as a UCD hereinafter).
In addition, the MAC messages include various messages for performing a request (REQ) function, a response (RSP) function, and an acknowledgement (ACK) function on various operations.
FIG. 4 shows a diagram for a frame structure of the wireless portable Internet system.
Referring to FIG. 4, frames include a downlink sub-frame and an uplink sub-frame depending on transmission directions. The vertical axis of the frame represents subchannel logical numbers, and the horizontal axis thereof denotes OFDMA symbol numbers.
The downlink sub-frame includes a preamble, a downlink MAP (DL-MAP), an uplink MAP (UL-MAP), and a plurality of downlink (DL) bursts. The DL bursts may not represent the channels or resources classified according to users, but they are classified according to transmission levels with the same modulation scheme or channel encoding. Further, the DL bursts can be provided for respective users.
The downlink MAP has offset information, modulation method information, and coding information on a plurality of users who use the same modulation method and channel coding, and allocates the resources to the users. Accordingly, the MAP has a feature of broadcast channels and requires strong robustness.
In the case of the uplink sub-frame, transmission is performed per user, and the uplink bursts have per-user information.
FIG. 5 shows a flowchart for establishing a connection process in the wireless portable Internet system.
Referring to FIG. 5, when an SS enters a base station's area in step S501, the BS establishes downlink synchronization with the SS in step S502. When the downlink synchronization is established, the SS acquires an uplink parameter in step S503. For example, the parameter can be included in a channel descriptor message according to the physical layer characteristics (e.g., useable burst profiles corresponding to the appropriate SNR (signal to noise ratio) levels).
A ranging process between the SS and the BS is performed in step S504. The ranging process for correcting timing, power, and frequency information between the SS and the BS performs an initial ranging process and a periodic ranging process after the initial ranging.
When the ranging process is finished, a negotiation on basic service provision capabilities for establishing connection between the SS and the BS is performed in step S505. When the negotiation on basic service provision capabilities is finished, the SS is authenticated in step S506 by using a device identifier including an MAC address and a certificate of the SS.
When the authentication for the SS is finished and a usage authorization on the wireless portable Internet is confirmed, a device address of the SS is registered in step S507, and an IP address is provided to the SS from an IP address management system such as a DHCP server to accordingly establish an IP connection in step S508.
The SS assigned with the IP address performs a connection-establishment process for data transmission in step S509.
The above-described wireless portable Internet system not only performs communication near a fixed location but also has mobility in the metropolitan level differing from the conventional wireless LAN communication systems, and hence, batteries are usually used to supply power to the SS, and the duration of the batteries is a major limitation of the usage time in the wireless portable Internet system.
Therefore, the wireless portable Internet system such as the IEEE 802.16e has proposed a sleep mode for reduction of battery power consumption. The sleep mode is a method for allowing a terminal to enter a sleep state during a sleep window, and reduce the SS's power consumption when no data to be transmitted to the SS is provided. After entering the sleep state, the SS performs no operation for data transmission during the sleep window, thereby saving the power consumption of the SS.
The SS is switched to a listening state each time the sleep window is terminated, and it checks whether data which stands by to be transmitted (to the corresponding terminal) during the sleep window are provided.
FIG. 6 shows a signal flowchart for a sleep mode operation in the wireless portable Internet system.
As shown, entering the sleep mode by the SS requires permission by the BS. The SS 111 attempting to enter the sleep mode establishes a sleep window to request a sleep mode from the BS 112 in step S601.
When receiving the sleep mode request, the BS assigns a sleep window to transmit a sleep mode approval to the SS in step S602.
When receiving the sleep mode approval, the SS enters the sleep mode for receiving no data at the sleep mode entering time M in step S603. When the initial sleep window is expired, the SS is switched to a listening mode to check whether data addressed to the SS (in a transmission standby state) are buffered from the BS during the sleep window in step S604.
In this instance, when no data addressed to the SS (in the transmission standby state) are buffered during the initial sleep window, the BS 112 establishes a message for indicating existence of data traffic to be 0 and transmits the same to the SS in step S605.
When it is determined that no data traffic is transmitted during the listening mode, the SS enters the sleep mode again in step S606. In this instance, the sleep window can be established to be equal to or longer than the initial sleep mode.
When data in the transmission standby state with respect to the SS 111 are provided during a second sleep window, the BS can buffer the data traffic in step S608, and existence of the buffered data are reported in the listening mode of the SS.
The BS 112 establishes a field corresponding to the message which indicates existence of data traffic to be a field (e.g., 1) for indicating the existence, or transmits a message which includes a list of basic CIDs which are identifiers of corresponding SSs to the SS in step S609. When receiving the message and checking that the data traffic to be transmitted to the SS 10 are found in the listening mode in step S607, the SS 111 terminates the sleep mode, enters an awake mode to receive the buffered data traffic, and performs data communication with the BS 20 in step S610.
The SS 10 proceeds to the sleep mode according to the sleep mode operation when there are no data to be transmitted, thereby preventing unnecessary power consumption.
FIGS. 7 and 8 show exemplified sleep windows in the conventional sleep mode.
FIG. 7 shows exemplified terminals operable by a power saving operation mode with a periodic sleep mode, and FIG. 8 shows a power saving mode operation with an exponentially increasing sleep window.
Referring to FIG. 7, a subscriber station SS1 (710) listens to a frame once for each N/4 frame, and a subscriber station SS2 (720) listens to a frame once for each N/2 frame.
Therefore, broadcast information which is needed to be listened to by the subscriber states SS1 and SS2 is broadcast once for each N/2 frame, and information which is needed to be transmitted for a specific subscriber station SS1 is broadcast by a subframe with a period of an N/4 frame.
However, the periodic power saving mode is easy to manage, but its power saving efficiency is not good because most of the data traffic is shown at a specific time (i.e., a burst characteristic), and periodic switching to the listening mode is inefficient for power saving in the data communication system such as the Internet.
Since the data traffic other than voice traffic has a burst characteristic and a long-range dependence as described above, it is desirable to exponentially increase the next sleep window when no data traffic in the transmission standby is provided in the listening mode.
As shown in FIG. 8, a subscriber station SS3 initially has a sleep window of an N frame, and it exponentially increases the sleep window such as to 2N, 4N, and 8N.
However, the case of exponentially increasing the sleep window is effective when the data traffic has a long-range dependence, but it increases complexity of the system since it must manage the sleep window and the listening interval for the respective subscriber stations.
Also, the power saving operation method shown in FIG. 8 is not efficient for traffic which has a very long interval and periodically appears.
As to the HIPERLAN/2 system, each SS enters the sleep state with a predetermined sleep window. In this instance, the sleep window is allowed during a frame time corresponding to a value of exponentiation of 2. The frame which corresponds to a listening interval of an SS having a shorter sleep window is superimposed on the frame which corresponds to a listening interval of an SS having a longer sleep window. For example, the listening interval of the SS which is in the sleep mode with the period of eight frames is superimposed on the listening interval of the SS which is in the sleep mode with the period of four frames, which advantageously prevents repetition of management compared to the method of individually managing the listening interval of each SS since the sleep windows of SSs are managed by grouping them.
However, recent transactions have reported that the Internet traffic has a long-range dependence or self-similarity, which represents that the burst characteristic of traffic is stronger and is long-lasting. Therefore, the above-noted method is very efficient when the traffic occurs with a fixed period, but the same may be inefficient when the self-similarity is strong in a like manner of the Internet traffic.
U.S. Pat. No. 5,758,278 (May 26, 1998) entitled “Method and apparatus for periodically reactivating a mobile telephone system clock prior to receiving polling signals” discloses a method which is only applicable to periodical reactivation, and finds an awaking time with less power consumption on the basis of periodicity to thus control the mobile telephone to efficiently awake before periodical polling signals are generated.
The transaction entitled “IEEE 802.16e Sleep Mode” published in IEEE 802.16e Session #24 Contribution, pp. 1 to 8 (Mar. 11, 2003) discloses required improvements to be supported by the IEEE 802.16a standards in order to support the mobility operation by reducing power consumption of the subscriber station. The transaction proposes a scheme for applying a typical burst characteristic of traffic, transmitting no packets in the off period in which no packets are generated to thereby control the subscriber station to be in the sleep mode and reduce power consumption of the subscriber station. In particular, the scheme controls the sleep window to be double and reduces the power consumption of the subscriber station when no traffic is found to be transmitted to the corresponding subscriber station, by considering that the traffic of the Ethernet and the Internet maintains the duration of the traffic when no traffic is provided.
That is, the transaction applies a sleep window update algorithm which uses an exponentially-increasing sleep window to increase the sleep window when no traffic is provided, thereby saving the power. However, the transaction fails to provide a method for grouping the subscriber stations and managing the grouped subscriber stations for the conventional case in which the initial sleep window has no relation to the final sleep window, thus deteriorating the efficiency of management and increasing the size of a signaling message.
The IEEE 802.16e system adopts a concept of exponentially increasing the sleep window in the operation of sleep mode. That is, on entering the sleep mode, the subscriber station checks the traffic during the sleep window, and when no traffic is found to be transmitted to the subscriber station, the subscriber station doubles the next sleep window and enters the sleep mode again.
The above-noted process is allowed in consideration of the self-similarity of Internet traffic, and hence, when no traffic is provided for a predetermined period, it is more probable that no traffic is provided for a longer period, and the efficiency of the sleep mode is increased.
In addition, the process for the subscriber station to enter the sleep mode with the initial sleep window, double the sleep window and enter the next sleep mode when no traffic occurs during the sleep mode is repeated until it comes to the final sleep window. The subscriber station enters the next sleep mode with the final sleep window value when the final sleep window is less than the doubled previous sleep window.
The above-described mechanism may generate the same effect as that of the sleep mode operable by the fixed sleep window when the initial sleep window is established to correspond to the final sleep window. However, the mechanism fails to arrange the listening intervals of the subscriber station operable by the sleep mode, and requires searching the subscriber stations in the sleep state for each frame in order to indicate traffic, and has a problem when a large volume of signaling messages for indication of traffic are applied to a specific frame.